Low-noise block down-converters (LNBs) are devices that may be used for satellite TV reception. Typically, they are mounted on a satellite dish for down-converting the received radio frequency (RF) signals. LNBs typically may include features such as a low-noise amplifier, a frequency mixer, a local oscillator and an intermediate frequency (IF) amplifier.
One example of a LNB is a quad LNB. A quad LNB has a single feed-horn that has four outputs that are connected to four different tuners. Each output responds to the tuners band and polarisation selection signals independently of the other outputs, and each output appears to the tuner to be a separate LNB. In devices of this kind, and also in other kinds of devices where it is necessary to selectively switch between different down-converted signals, it is known to provide a switch matrix of the kind shown in FIGS. 1 and 2.
The switch matrix 2 shown in FIG. 1 includes eight transmission lines 16, 18. The transmission lines 18 are each connected to inputs 4, while transmission lines 16 are each connected to outputs 6. The transmission lines 16, 18 are arranged in a grid or matrix and are provided with shunt switches 8 which can be opened or closed selectively to connect the transmission lines of the inputs 4 to the transmission lines of the outputs 6. In this way, each output can selectively output the down-converted RF signal received at any one of the inputs 4.
A key requirement for down-converters that are used in, for example, satellite TV reception is that the receive channel for each tuner is not polluted by the other channels and/or is not influenced by the channels selected by any of the other tuners of the system. With reference to FIG. 1, it is therefore beneficial if the transmission lines 16, 18 of the switch matrix 2 are well isolated from each other, specifically when they are not connected together by one of the shunt switches 8. However, it can be seen from FIG. 1 that the transmission lines 16 of the outputs 6 cross over the transmission lines 18 of the inputs 4 in a number of places, corresponding to the locations of the shunt switches 8. Each cross-over point can degrade isolation performance of the device due to capacitive and/or magnetic coupling between the transition lines 16, 18. The number of cross-over points that are relevant to the degree of isolation between the transmission lines 16, 18 for a switch matrix 2 of the kind shown in FIG. 1 is influenced by the switching configuration of the matrix. This is explained below in FIGS. 2A-2D.
In each of FIGS. 2A-2D, the configuration or state of the shunt switches 8 is indicated as being either closed (see the dots labelled 12) or open (see the crosses labelled 14).
In the example of FIG. 2A, the shunt switches are configured such that each output 6 is connected to a different input 4. The transmission line 16 connected to each respective output 6 in this example crosses over three of the transmission lines 18. Also, the transmission line 18 connected to each output 6 (through a closed shunt switch 12) itself crosses over three of the transmission lines 16. Therefore, a total of six cross-over points contribute to unwanted coupling that may adversely affect the signal at each output 6.
In FIG. 2B, two of the outputs are connected to a first common one of the inputs, while two of the outputs are connected to a second common input. The transmission lines 16 connected to the top two outputs in FIG. 2B both cross over three of the transmission lines 18 and are connected together via the transmission line 18 connected to the first common input, which itself crosses over the two other transmission lines 16. Similarly, the transmission lines 16 connected to the bottom two outputs in FIG. 2B both cross over three of the transmission lines 18 and are connected together via the transmission line 18 connected to the second common input, which itself crosses over the two other transmission lines 16. In this example therefore, a total of eight cross-over points contribute to unwanted coupling that may adversely affect the signal at each output 6. The amount of unwanted coupling between the transmission lines 16, 18 in this switching state may therefore be higher than in the switching state shown in FIG. 2A.
In FIG. 2C, three of the outputs are connected to a first common input while the fourth output is connected to a different input. The transmission lines 16 connected to the top three outputs in FIG. 2C each cross over three of the transmission lines 18 and are connected together via the transmission line 18 connected to the first common input, which itself crosses over one of the other transmission lines 16. Accordingly, a total of ten cross-over points contribute to unwanted coupling that may adversely affect the signal at these three outputs 6. The transmission line 16 connected the fourth output 6 in this example crosses over three of the transmission lines 18. Also, the transmission line 18 connected to the fourth output 6 (through a closed shunt switch 12) itself crosses over three of the transmission lines 16. Therefore, a total of six cross-over points contribute to unwanted coupling that may adversely affect the signal at the fourth output 6. The amount of unwanted coupling between the transmission lines 16, 18 in this switching state may therefore be higher than in the switching states shown in FIGS. 2A and 2B. Note that each output also experiences a different amount of unwanted coupling.
In the example of FIG. 2D, all of the outputs are connected to the same common input. The transmission lines 16 connected to all four outputs in FIG. 2D each cross over three of the transmission lines 18 and are connected together via the transmission line 18 connected to the common input. Accordingly, a total of twelve cross-over points contribute to unwanted coupling that may adversely affect the signal at each output 6. The amount of unwanted coupling between the transmission lines 16, 18 in this switching state may therefore be higher than in the switching states shown in any of FIGS. 2A to 2C.
To summarise, when using a switch matrix 2 of the kind shown in FIGS. 1 and 2, the amount of unwanted coupling between the transmission lines is dependent upon the switching state of the matrix 2. In some switching states, the amount of coupling between the transmission lines 16, 18 can be high. Also, in some switching states, some ports may experience a different amount of unwanted coupling than the other ports. Because the loading conditions presented to the output ports of the switch matrix 2 is dependent upon the switching state of the matrix, switch matrices of the kind shown in FIGS. 1 and 2 are rarely used in radio frequency (RF) designs.